Seamless 4 camera setup
You can achieve this result by duplicating your first Camera and than rotating it:



I reccomend not using seamless rotation. If you have a small overlapping part it is easier to match it up in the compositor.

To match the to renders up you can use the translate node:



The first part of the image.  



The second part off the image.  



Combined.

First, you take your to images. For demonstartion purposes I use to files on my computer. One file can also be the rendered image. Than scale the images to the right size. Make sure that your scene has the size of the combined image.
Than you need to add these to images together. To get the combined size I reccomend using a mask of scene size. Use a "translate" node to get the images into the right position. Using half your image height is a good starting point. Add the first image on top of the mask with an "add" node. Add the second image with a "lighten" node.
If your images don't overlay you can also use an "add" node.
If your images overlay match up the second image to the first one with the "translate" node. Also add another scale node to make the image in  this case a perfect square. This last step is only neccessary if you have overlaying images.  

I personaly reccomend doing the matching up in an image editor like Gimp ore Photoshop. It will save you a lot of time.